This invention relates to resid hydrotreating and, more particularly, to a process for minimizing solids formed during resid hydrotreating.
In the past, spiraling oil costs, extensive price fluctuations, and artificial output limitations by the cartel of oil producing countries (OPEC) have created instability and uncertainty for net oil consuming countries, such as the United States, to attain adequate supplied of high-quality, low-sulfur, petroleum crude oil (sweet crude) from Norway, Nigeria, and other countries at reasonable prices for conversion into gasoline, fuel oil, and petrochemical feedstocks. In an effort to stabilize the supply and availability of crude oil at reasonable prices, Amoco Oil Company has developed, constructed, and commercialized extensive, multimillion dollar refinery projects under the Second Crude Replacement Program (CRP II) to process poorer quality, high-sulfur, petroleum crude oil (sour crude) and demetallate, desulfurize, and hydrocrack resid to produce high-value products, such as gasoline, distillates, catalytic cracker feed, metallurgical coke, and petrochemical feedstocks. The Crude Replacement Program is of great benefit to the oil consuming nations by providing for availability of adequate supplies of gasoline and other petroleum products at reasonable prices while protecting the downstream operations of refining companies.
During resid hydrotreating, such as under Amoco Oil Company's Crude Replacement Program, resid oil is upgraded with hydrogen and a hydrotreating catalyst to produce more valuable lower-boiling liquid products. Undesirably, carbonaceous solids are formed, however, during resid hydrotreating. These solids have been characterized as multicondensed aromatics which form and precipitate from cracking of the side chains of asphaltenes. Asphaltenic solids are substantially insoluble in hexane, pentane, and in the effluent hydrotreated product oil. The solids become entrained and are carried away with the product.
Alphaltenic solids tend to stick together, adhere to the sides of vessels, grow bigger, and agglomerate. Asphaltenes are more polar and less soluble than the residual oil feedstock.
Asphaltenic carbonaceous solids are produced as a reaction by-product during fixed bed hydrotreating, hydrovisbreaking, and ebullated bed hydrotreating (expanded bed hydrotreating). During ebullated bed hydrotreating, the ebullating hydrotreating catalyst fines serve as a nucleus and center for asphaltene growth. The situation becomes even more aggravated when two or more hydrotreating reactions are connected in series as in many commercial operations. In such cases, solids formed in the first reactor not only form nucleation sites for solids growth and agglomeration in the first reactor, but are carried over with the hydrotreated product oil into the second reactor, etc., for even larger solids growth and agglomeration.
The concentration of carbonaceous solids increases at more severe hydrotreating conditions, at higher temperatures and at higher resid conversion levels. The amount of carbonaceous solids is dependent on the type of feed. Resid conversion is limited by the formation of carbonaceous solids.
Solids formed during reside hydrotreating cause deposition and poor flow patterns in the reactors, as well as fouling, plugging, and blocking of conduits and downstream equipment. Oils laden with solids cannot be efficiently or readily pipelined. Hydrotreating solids can abrade valves and other equipment, and can build up insulative layers on heat exchange surfaces reducing their efficiency. Buildup of hydrotreated solids can lead to equipment repair, shutdown, extended downtime, reduced process yield, decreased efficiency, and undesired coke formation.
Over the years, a variety of processes and solvents, hydrogen donors, and other fluids have been suggested for various refining operations, such as for upgrading oil or reducing hydrotreated solids. Typifying some of these prior art processes, solvents, hydrogen donors, and other fluids are those described in U.S. Pat. Nos. 2,879,224; 3,579,436; 3,681,231; 4,137,149; 4,434,045; 4,451,354; 4,525,267; 4,457,831; 4,485,004; and 4,495,060. These prior art processes, solvents, hydrogen donors, and other fluids have met with varying degrees of success.
It is, therefore, desirable to provide an improved process for minimizing solids from resid hydrotreating which overcomes most, if not all, of the above problems.